To highlight a feature, call the highlight method of the feature's corresponding layerView with the feature or its objectID as parameter. When passing features as parameters, they need to have an objectID.

view.whenLayerView(layer).then(function(layerView) {
// if a feature is already highlighted, then remove the highlightif (highlightSelect) {
highlightSelect.remove();
}
// set the highlight on the first feature returned by the query
highlightSelect = layerView.highlight(feature);
});